Add an example of stdgpu::unordered_map

[ykkawana]

It took me a bit while to grab how to use unordered_map, so I made this simple example code.

[stotko]

Thank you. I have a few remarks:

• container_kernel.cu contains a similar example on how to use the stdgpu::unordered_set within a kernel.
• There is also a similar code snippet in the examples section of the README.
• In your example, there are many calls to cudaMalloc, cudaFree and cudaMemcpy. I would prefer stdgpu's function, i.e. createDeviceArray etc.

Therefore, I think it is a good idea to add an explicit example for stdgpu::unordered_map. Please adapt it to match the other examples and the general coding style of the library.

[stotko]

More examples have been added by #173. In addition, much effort has been much put into improving the documentation, so I think it should be easier now for users to get an idea of the library.

If you still have suggestions on how to improve certain aspects of the library, please open an issue. Thanks again for your contribution. Closing.